Water supply is an important indication of modernization of a society. It not only influences general public health, industrial developments and commercial prosperity but also affects people's daily life. Therefore, supplying high-quality, safe and sufficient drinking water is one of a government's most important tasks. Conventional water treatments depend on good-quality raw water and stable purifying facilities to provide hundreds of thousands tons of drinking water that meets the drinking water standards. However, heavy rainfall after natural disasters such as earthquake and hurricane and human activities such as over-development on hillsides and poor soil-and-water conservation may result in surface runoff containing tremendous amount of suspended particles in streams and subsequently lakes and dams. The extra particle impurity causes high turbidity in raw water sources, which hinders the purifying functions in water treatment facilities and often temporarily disables water supply system in severe conditions.
Turbidity in surface water is usually caused by sands, clay, silt, soil particles and other colloidal impurity. Surface water movement after heavy rainfall washes soil particles from hillsides and disturbs sediments in the waterways results in a significant increase in the level of turbidity. Removal of water turbidity is somehow difficult because the fine particles are most likely too tiny to settle effectively in the water treatment processes. The rapid sand filtration system includes: coagulation, flocculation, gravity deposition, rapid sand filtration, disinfection by adding chlorine, etc. The efficiency of the rapid sand filtration system is dependent upon inflow water turbidity. When water turbidity exceeds 2,000 nephelometric turbidity units (NTU), most water treatment facilities may experience clog malfunction in rapid sand filtration process. If a rapid sand filtration pool is frequently clogged, back-wash is frequently required to restore its filtration function. However, high frequency of back-wash significantly decreases the water quality.
In a rapid sand filtration system, coagulants react with suspended particles to form flocs. The flocs then settle in sedimentation basin so as to avoid clogging the sand-filtering pool. However, chemical coagulation produces large amounts of sludge and reduces the density of the flocs, which hinders the deposition thereof. After rainfall, the turbidity of raw water may reach tens of thousands NTU. Traditional water purifying process is unable to process such high-turbidity raw water by coagulation or gravity deposition. The malfunction of water treatment plants caused by high turbid raw water will greatly affect water quality and may temporarily disrupt the water supply if the turbidity is extremely high. Water supply is a civil necessity. Lack of water supply is detrimental to people's health and local economy. Therefore, it is desirable to provide a suitable water clarifying method to resolve the problem of high turbidity in water after heavy rainfall, especially in the event of hurricanes.
The main characteristic of highly turbid raw water is excessive amount of suspended particles. The level and nature of the turbidity depends upon the types and sizes of particles over which the water has run and the velocity of the water. In flowing water, suspended particles mix well with water and do not settle easily. When the water becomes still, larger and heavier suspended particles sink quickly, while the smaller and lighter ones remain suspended. Deposition of such fine particle can be achieved only when the water is absolutely still and it may take weeks or even months for the water to become clear. It is well known that the need for water supply in a modern society increases every year due to population and economy growth. Highly turbid water requires large tracts of land for the installation of extra sedimentation and filtration facilities, which are difficult to acquire especially in metropolitan areas. Further, the efficiency of the rapid sand filtration system in producing good quality and sufficient quantity of drinking water cannot be achieved without appropriate water turbidity pre-treatment process.
Alternatively, centrifugal methods were once applied to remove extra water turbidity. However, the centrifugal method consumes huge amount of electricity and requires large spaces while its turbidity removal efficiency is very low. Currently, most water treatment plants add large amount of chemical coagulants to raw water to accelerate the floc formation and effectively remove water turbidity. Larger flocs associated with tiny particles in water tend to settle faster in the sedimentation tank to clarify the water. However, chemical coagulating agents produce large amount of sludge, which in turn becomes another pollution problem.
Accordingly, an apparatus and/or method that removes water turbidity by physically accelerating the deposition of particles in water, relieves the impact of high-turbidity water and overcomes the defects of conventional water treatment problems, such as consumption of large amount of electricity, low deposition efficiency, sludge formation created by chemical coagulants, is always welcome.